Question

why does the FileSystemWatcher fire twice? Is there an easy way to fix it? Surely if I update or edit the text file it should only fire once?

this link here http://weblogs.asp.net/ashben/archive/2003/10/14/31773.aspx says

1. Events being raised twice – An event will be raised twice if an event handler (AddHander FSW.Created, AddressOf FSW_Created) is
   explicitly specified. This is because, by default, the public events
   automatically call the respective protected methods (OnChanged,
   OnCreated, OnDeleted, OnRenamed). To correct this problem, simply
   remove the explicit event handler (AddHandler …).

What does “remove the explicit event handler” mean?

Imports System.IO

Public Class Form2

    Private Sub FileSystemWatcher1_Changed(ByVal sender As System.Object, ByVal e As System.IO.FileSystemEventArgs) Handles FileSystemWatcher1.Changed

        'this fires twice
        MessageBox.Show("test")

    End Sub

    Private Sub Form2_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load

        FileSystemWatcher1.Path = "C:\Users\c\Desktop\test\"
        FileSystemWatcher1.NotifyFilter = NotifyFilters.LastAccess Or NotifyFilters.LastWrite Or NotifyFilters.FileName Or NotifyFilters.DirectoryName Or NotifyFilters.CreationTime

        FileSystemWatcher1.IncludeSubdirectories = False
        FileSystemWatcher1.Filter = "text.txt"

    End Sub

End Class

Answer 1

Update:

I have come up with 2 solutions. One uses Threads, and the other doesn’t. Take your pick :-).

Without threading:

Imports System.IO

Public Class Form1
    Private Sub FileSystemWatcher1_Changed(ByVal sender As System.Object, ByVal e As System.IO.FileSystemEventArgs) Handles FileSystemWatcher1.Changed
        Dim watcher As System.IO.FileSystemWatcher = sender
        watcher.EnableRaisingEvents = False

        'Do work here while new events are not being raised.
        MessageBox.Show("Test")

        watcher.EnableRaisingEvents = True 'Now we can begin watching for new events.

    End Sub

    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load

        FileSystemWatcher1.Path = "C:\Users\c\Desktop\test"
        FileSystemWatcher1.NotifyFilter = NotifyFilters.LastWrite
        FileSystemWatcher1.IncludeSubdirectories = False
        FileSystemWatcher1.Filter = "test.txt"


    End Sub

    Private Sub FileSystemWatcher_OnChanged(ByVal sender As System.Object, ByVal e As System.EventArgs)

    End Sub

End Class

This solution (without threading), sets the watcher.EnableRaisingEvents to False. It is after this point where you would normally process whatever files are affected (or changed). It then sets the EnableRaisingEvents back to True after your work is done.

With threading:

Imports System.IO

Public Class Form1
    Private Sub FileSystemWatcher1_Changed(ByVal sender As System.Object, ByVal e As System.IO.FileSystemEventArgs) Handles FileSystemWatcher1.Changed
        FileSystemWatcher1.EnableRaisingEvents = False
        Threading.Thread.Sleep(250)
        FileSystemWatcher1.EnableRaisingEvents = True


        MessageBox.Show("test")


    End Sub

    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load

        FileSystemWatcher1.Path = "C:\Users\c\Desktop\test"
        FileSystemWatcher1.NotifyFilter = NotifyFilters.LastWrite
        FileSystemWatcher1.IncludeSubdirectories = False
        FileSystemWatcher1.Filter = "test.txt"


    End Sub

    Private Sub FileSystemWatcher_OnChanged(ByVal sender As System.Object, ByVal e As System.EventArgs)

    End Sub

End Class

This solution, although a bit hacky, does work. It disables checking for new changes/events for 250ms and then re-enables checking, based on the assumption that you won’t been needing to check for a change every 250ms. I have tried almost everything that I could think of to get a real solution for you but this will work in the meantime.